It is beneficial for operators of satellite communications to deny adversaries from eavesdropping on their transmissions. One way adversaries eavesdrop is by using a high gain antenna to receive a small amount of stray radio frequency radiation emitted from the side-lobes or back-lobes of satellite communication terminal antenna. The adversaries may then use a spectrum analyzer to monitor the transmit signal power spectral density over time in order to derive useful intelligence. For example, an adversary could associate an increase in power spectral density with greater activity on the terminal.
In one prior art system and method for attempting to deny adversaries from eavesdropping on satellite transmission, as provided in U.S. Pat. No. 7,136,621, code division multiple access (CDMA) signals are transmitted from the satellite communication terminal and intentionally spread in frequency using direct sequence spread spectrum modulation. This reduces the transmit power spectral density thereby lowering the probability of intercept and detection by an eavesdropper. However, if the eavesdropper gets sufficiently close to the transmitting satellite communication terminal, or if the eavesdropper uses a big enough antenna, it is possible to detect and monitor transmission activity on the terminal. The transmitted power and power spectral density may be directly proportional to the data rate of traffic transmitted from the terminal, so the eavesdropper may monitor terminal activity by measuring fluctuations in received power.
In another prior art system and method for attempting to deny adversaries from eavesdropping on satellite transmissions, the satellite communication terminal may be operated at a fixed transmit power and data rate which meets the peak power and data rate demand of the satellite communication terminal. Using this constant activity system and method, an eavesdropper may not detect change in terminal activity. However, this may be inefficient because the average demand for satellite communication terminal data rate is typically a small fraction of the peak demand constant rate.
Another prior art system and method for attempting to deny adversaries from eavesdropping on satellite transmissions in time division multiple access (TDMA) systems is to randomly assign time slots to terminals to randomize the duration and interval between transmission bursts. Without randomizing transmissions, an eavesdropper may determine peak periods of activity, identify unusual or unexpected activity spikes, and identify locations of remote terminals that have remained quiet for a period of time and suddenly experience increased traffic volumes. The adversary may subsequently extrapolate timing, location, and scale of the communications.
Another approach may use frequency division multiple access (FDMA), which may work similarly to the time division multiple access approach, except that the random assignment of unused satellite capacity may occur in the frequency domain rather than in the time domain. Alternatively the allocation may be to a fixed number of frequency slots with null data transmitted in the excess slots. However, this method may be inefficient due to the typically large difference between the actual data rate demand and the peak resource allocation, thereby reducing the useful aggregate data rate of the communication resource.
A system and method is needed that is both effective at obfuscating signal transmissions and efficient in use of satellite resources.